Microcontact printing is a technique for forming patterns of organic monolayers with micrometer and submicron lateral dimensions. It offers experimental simplicity and flexibility in forming certain types of patterns. It relies on the remarkable ability of self-assembled monolayers of long-chain alkanethiolates to form on gold and other metals. These patterns can act as nanometer resists by protecting the supporting metal from corrosion by appropriately formulated etchants, or, can allow for the selective placement of fluids on hydrophilic regions of the pattern. Patterns of self-assembled monolayers having dimensions that can be less than 1 .mu.m are formed by using the alkanethiol as an "ink", and by printing them on the metal support using an elastomeric "stamp". The stamp is fabricated by molding a silicone elastomer using a master prepared by optical or X-ray microlithography or by other techniques.
Microcontact printing of patterned self-assembled monolayers brings to microfabrication a number of new capabilities. First, microcontact printing makes it possible to form patterns that are distinguished only by their constituent functional groups; this capability permits the control of surface properties such as interfacial free energies with great precision. Second, because microcontact printing relies on molecular self-assembly, it generates a system that is (at least locally) close to a thermodynamic minimum and is intrinsically defect-rejecting and self-healing. Simple procedures, with minimal protection against surface contamination by adsorbed materials or by particles, can lead to surprisingly low levels of defects in the final structures. The procedure can be conducted at atmospheric pressure, in an unprotected laboratory atmosphere. Thus, microcontact printing is especially useful in laboratories that do not have routine access to the equipment normally used in microfabrication, or for which the capital cost of equipment is a serious concern. Third, the patterned self-assembled monolayers can be designed to act as resists with a number of wet-chemical etchants.
Working with liquid etchants suffers from the disadvantages of handling solvents and disposing of wastes, but also enjoys substantial advantages: a high degree of control over contamination of surfaces; reduced damage to the substrate from energetic interactions with atoms or ions; the ability to manipulate complex and sensitive organic functionalities. Because the self-assembled monolayers are only 1-3 nm thick, there is little loss in edge definition due to the thickness of the resist; the major determinants of edge resolution seem to be the fidelity of the contact printing and the anisotropy of etching the underlying metal. In the current best cases, features of size 0.2 .mu.m can be fabricated; edge resolution in systems showing this resolution in thickness is less than 50 nm.
Gels are cross-linked networks of polymers swollen with a liquid. Softness, elasticity, and the capacity to store a fluid make gels unique materials, and soft and gentle materials are beginning to replace some of the hard mechanical materials in various industries.
Due to the cross-linking, various properties of individual polymers become visible on a macroscopic scale. The polymer network changes its volume in response to a change in environment: temperature, solvent composition, mechanical strain, electric field, exposure to light, pH, salt concentration, etc. Advances in Polymer Science, ed. K. Dusek, Vol. 109, p. v (Springer-Verlag New York 1993); S. Saito, pp. 207-232, Id.; M. Shibayama and T. Tanaka, pp. 1-62, Id.; Y. Osada, et al., pp. 82-87, Scientific American (May 1993); Y. Osada and J. Gong, Prog. Polym. Sci., vol. 18, pp. 187-226 (Great Br. 1993); Irie, M., pp. 49-65 in Advances in Polymer Science, ed. K. Dusek, Vol. 110 (Springer-Verlag New York 1993); E. Kokufuta, pp. 157-77, Id.; T. Okano, pp. 179-197, Id., all incorporated by reference.
Hydrophilic gels in aqueous solution have been the most widely studied, but almost any polymer can be cross linked to form a gel which will swell in a sufficiently good quality solvent. The three-dimensional network is stabilized by cross links which may be provided by covalent bonds, physical entanglements, crystallites, charge complexes, hydrogen bonding, van der Waal's or hydrophobic interactions. Gels have many technologically important roles in chemical separations, biomedical devices and absorbent products, to name a few areas. The properties that make gels useful include their sorption capacities, swelling kinetics, permeabilities to dissolved solutes, surface properties (e.g., adhesiveness), mechanical characteristics, and optical properties. The single most important property of a gel is its swelling degree, since most of the properties are directly influenced by this. S. H. Gehrke, p. 85, in Advances in Polymer Science, ed. K. Dusek, Vol. 110 (Springer-Verlag New York 1993).
"Responsive" polymer gels are materials whose properties, most notably their solvent-swollen volumes, change in response to specific environmental stimuli including temperature, pH, electric field, solvent quality, light intensity and wavelength, pressure, ionic strength, ion identity, and specific chemical triggers, like glucose. S. Saito, pp. 207-232; M. Shibayama and T. Tanaka, pp. 1-62. The property which often changes the most dramatically is the swollen volume. These changes may occur discontinuously at a specific stimulus level (a phase transition), or gradually over a range of stimulus values. All of these changes are reversible with no inherent limit in lifetime.
Gels have been employed as chemical sensing surfaces, for example, in conjunction with fiber-optic systems, or elaborate mechanical or electrode systems. These systems are often quite elaborate, and suffer either from lack of flexibility or expense, or both. For example, U.S. Pat. No. 5,436,161 to J. Bergstrom, et al., discloses a matrix coating for surface plasmon resonance detection, to be used with a rigid dielectric material, such as a glass plate.
The information-carrying capacity of light provides an elegant method for detecting and displaying information in a way that is readily interpreted by a human. Sensors that visibly change color in response to a surface antibody-antigen binding reaction are already commercially available. An example of such a device, based on thin film interference, is the group B streptococcal antigen detector made by Biostar.TM. [G. R. Bogart, et al., "Devices and methods for detection of an analyte based upon light interference," U.S. Pat. No. 5,482,830, (Assignee: Biostar, Inc. Boulder, Colo.)]. Another example of a very simple optical-based sensor is where a Bragg reflector expands in the presence of water to change the reflected wavelength. The detection and display components both device are integrated so that an electronic display (with associated power supply and processing circuit) is not needed. However, that sort of detection device is suitable only for a narrow range of sensor applications. There is a need for a sensor technology platform that can be slightly modified to accommodate a wide range of stimuli and sensing conditions. There is a need, therefore, for a simple sensing system that takes full advantage of the responsive properties of gels, but which is flexible, easy to use, and preferably, disposable.